The Year in Science: Technology 1997

Motion is the Root of All EvilSuppressed Roar

Suppressed Roar Supersonic jets aren’t just noisy when they boom through the sound barrier. Even at takeoff, the fuel exhaust shoots out the back at supersonic speeds, and when it hits the surrounding air, it creates shock waves that make a fearsome racket. Last January , aeronautical engineer Dimitri Papamoschou of the University of California at Irvine received a patent on a scheme to muzzle supersonic-jet engines. The key, Papamoschou realized, was to do something about the turbulent eddies that build up around supersonic exhaust. Each little eddy behaves like a bullet, he says. Since they move at supersonic speeds, shock waves form around them. A cylinder of metal surrounding the exhaust would muffle the sound, but would also be impractically heavy. Instead, Papamoschou’s idea is to create a cone of hot air that moves more slowly than the exhaust, but faster than the ambient air—and thus acts as a buffer between the two. In his scheme, air expelled from the engine’s fan is speeded up with additional fans and then pumped out around the exhaust pipe. Since no volume of air moves against another at supersonic speed, shock waves never form. In tests on scale models, the air shroud cut the noise of the engine by about 90 percent.